With regard to a related-art air-conditioning apparatus, Patent Literature 1 discloses an example of a defrosting method for a heat exchanger. In the configuration of the air-conditioning apparatus disclosed in Patent Literature 1, a compressor which compresses refrigerant into high-temperature high-pressure gas, a four-way valve which switches the flow of refrigerant, an indoor-side heat exchanger, an expansion valve functioning as a decompressing unit and an outdoor-side heat exchanger are connected by refrigerant pipes. Furthermore, a bypass pipe passage is provided to connect a portion between the expansion valve and the outdoor-side heat exchanger to a discharge side of the compressor, and an opening/closing valve is provided at the bypass pipe passage.
Next, the operation of the air-conditioning apparatus of Patent Literature 1 will be described. During a heating operation, high-temperature, high-pressure refrigerant discharged from the compressor is caused to flow into the indoor-side heat exchanger by the four-way valve, and is condensed in the indoor-side heat exchanger. In the indoor-side heat exchanger, gas refrigerant is cooled to change into condensed liquid. The condensed liquid is adiabatically expanded at the expansion valve to change into low-pressure refrigerant. The low-pressure refrigerant is heated and evaporated in the outdoor-side heat exchanger to change into low-pressure gas. The low-pressure gas passes through the four-way valve and is sucked into the compressor.
When the amount of heat removed from the outdoor-side heat exchanger to the refrigerant circuit is reduced as an outdoor temperature drops, and an evaporation temperature drops to be lower than or equal to the zero temperature, frost starts to form on the outdoor-side heat exchanger. Thereby, when the function of removing heat is deteriorated, and an input-pipe temperature of the outdoor-side heat exchanger further drops, the air-conditioning apparatus starts a defrosting operation. During the defrosting operation, the opening/closing valve provided at the bypass pipe passage is opened. Thus, after flowing out of the compressor, high-temperature, high-pressure gas passes through the bypass pipe passage, enters the outdoor-side heat exchanger, and melts the frost adhering to the surface of the outdoor-side heat exchanger. This is a hot-gas defrosting operation. After the defrosting operation is ended, the opening/closing valve is closed, and the operation of the air-conditioning apparatus is changed back to the heating operation.
The air-conditioning apparatus disclosed in Patent Literature 1 is of a hot-gas defrosting type. Thus, liquid backflow, which is a phenomenon in which liquid refrigerant condensed in the outdoor-side heat exchanger during the defrosting returns to the compressor, often occurs, and as a result the inner temperature of the compressor drops. In particular, in the case where the inside of the compressor is of a low-pressure shell type, a large amount of refrigerant is dissolved in refrigerating machine oil for lubrication of the inside of the compressor. As a result, the viscosity of the refrigerating machine oil is reduced to cause insufficient lubrication. Thus, the reliability of a bearing of the compressor is also reduced.
An example of a measure for reducing the liquid backflow is disclosed in Patent Literature 2. In a method disclosed in Patent Literature 2, a bypass circuit which connects a refrigerant-outlet side of an outdoor-side heat exchanger during a defrosting operation and a suction port of a compressor to each other, and a heater and a capillary tube provided at the bypass circuit are provided in the air-conditioning apparatus, and when the amount of liquid refrigerant returning to the compressor is large, the heater heats the capillary tube to gasify the refrigerant. In the method disclosed in Patent Literature 2, it is necessary to provide the bypass circuit, the capillary tube and the heater, thus increasing the cost. In methods disclosed in Patent Literature 3 and Patent Literature 4, a bypass circuit which connects a discharge port of a compressor and an outdoor heat exchanger is provided, and the operation frequency of the compressor is increased in stages from the start of a defrosting operation. Also, in the methods disclosed in Patent Literature 3 and Patent Literature 4, it is necessary to provide the bypass circuit, thus increasing the cost.
As another measure for restricting the liquid backflow, Patent Literature 5 discloses a method in which a sensor for detecting a suction temperature at a compressor is provided, and when a suction pressure (temperature) is reduced, the operation frequency of the compressor is reduced. Patent Literatures 6 and 7 disclose methods in which when a defrosting operation is performed, the opening degree of an expansion valve provided between an indoor heat exchanger and an outdoor heat exchanger is controlled such that the degree of superheat of gas discharged from a compressor falls within a predetermined range. Patent Literature 8 discloses that after the operation to be performed is switched from a heating operation to a defrosting operation, the operation frequency of a compressor is increased when the temperature of refrigerating machine oil in the compressor becomes higher than or equal to a predetermined value, whereas the operation frequency of the compressor is reduced when the temperature of the refrigerating machine oil falls below the predetermined value.